The invention relates to a photomultiplier tube and particularly to a structure for reducing the dark current of the tube.
In certain photomultiplier tube applications operational instability may occur when the anode current level abruptly changes due to changes in the input signal. In such instances, it has been noted that a conductive pattern such as that disclosed in U.S. Pat. No. 3,873,867, to Girvin, issued Mar. 25, 1975, may be used to focus the electrons so that they will not impinge upon the insulating support spacers which hold the dynodes of the tube. The conductive pattern is intended to prevent the support spacers from charging under electron bombardment. By reducing the charging of the spacers the operational instability condition commonly known as "hysteresis" is prevented. The Girvin patent suggests that a conductive coating consisting of molybdenum material applied by a silk screening technique may be deposited on the ceramic spacer. Alternatively, it is also known that other materials such as aluminum or nickel may also be used. The conductive pattern is generally tied to the same potential as the first dynode; however, other potentials between first dynode potential and anode potential may also be used.
The aforementioned conductive coating generally has a resistance in the neighborhood of a few ohms per square and typically a conductive pattern having a resistance of one ohm per square is produced by the silk screening process. Tubes having the aforedescribed conductive pattern, which is fixed at or near the potential of the first dynode, are prone to exhibit excessive dark current when operated near the maximum operating voltage. This phenomenon is believed due to the fact that the conductive pattern which extends along the electron path from the first dynode to the anode creates a high electric field in the neighborhood of the last dynode adjacent to the anode. The electric field causes luminescence in the ceramic which feeds light back to the photocathode to increase the dark current by generating a spurious input signal. This problem is especially severe in the so called "tea-cup" photomultiplier tube used for scintillation counting. The tea-cup photomultiplier tube must pass a stringent dark current test at 1500 volts. A number of tea-cup photomultiplier tubes having silk-screened conductive nickel coatings disposed on chrome oxide coated support spacers have been unable to meet the dark current requirement. The chrome oxide, it has been found, quenches some of the electric field induced luminescence; however, the amount of light fed back to the photocathode is still sufficient to create excessive dark current within the tube. As a consequence, it is necessary to bake the photomultiplier tube at an elevated temperature in order to reduce the dark current. The baking process has an undesirable side effect in that it tends to reduce the cathode sensitivity of the tube and degrade the pulse height resolution of the tube while reducing the dark current. It is thus desirable to reduce the dark current by eliminating or reducing the high electric field that is present near the anode end of the conductive pattern that is formed on the support spacers.